Resistance-capacitance type oscillator



April 6, 1948. I J, c, DUNN 2,439,245

. RESISTANCE-CAPACITANCE TYPE OSCILLATOR Filed June 2, 1945 r /Z I W54 -EZ 6 K62 l I A 1P2 d- 9 Il I /5 M /7 ib/fifa/me 6ou o /e1 .9

iatented Apr. 6 1943 T OFFICE RESISTANCE-CAPACITANCE TYPE OSCILLATOR James C. Dunn, Philadelphia, Pa., assignor, by mesne assignments, to Philco Corporation, Philadelphia, Pa., a corporation of Pennsyl-- vania 'Application June 2, 1945, Serial No. 597,226

This invention relates to vacuum tube circuits, and more particularly to circuits employing feedback connections.

The principal object of the invention is to provide a novel feedback arrangement in a vacuum tube circuit, which arrangement may be employed to adapt the circuit for use either as an audio oscillator or as a selective audio amplifier.

A more specific object of the invention is to provide a novel feedback arrangement employing a bridged T network.

One aspect of the invention, therefore, has to do with the provision of a novel audio oscillator circuit. Conventional resistance-capacitance tuned audio oscillators have generally required at least two ganged, matched, variable resistors or condensers in order to provide for continuous variation of frequency over the operating frequency range. In its application to an audio oscillator, the present invention has for a further object the provision of a novel audio oscillator circuit in which continuous variation of frequency over a predetermined range may be efeffected by means of a, single variable element.

As applied to the purpose of audio amplification, the invention has for a further object the provision of a novel selective audio amplifier having a sharp cut-off characteristic for frequencies above and below the desired operating frequency.

Other objects and features of the invention will be apparent as the description proceeds. In the accompanying drawing, u

Fig. 1 is a diagrammatic illustration of a tuned audio oscillator embodying the invention; and Fig. 2 is a similar illustration of a selective audio amplifier embodying the invention.

Referring to Fig. 1, the audio oscillator circuit illustrated comprises a conventional audio amplifier represented generally by the broken line rectangle l, and a novel feedback arrangement which will be described hereinafter. While the audio amplifier may be of any suitable form, it

is preferred to employ at least a two-stage amplifier, as illustrated, comprising vacuum tubes V1 and B2 which may be either pentodes, as shown, or triodes with appropriate circuit modifications. The input tube V1 may be provided with the usual screen grid resistor 2 and associated by-pass condenser 3, and the usual plate resistor 4. The output tube V2 may be provided with the usual grid resistor 6 and may be coupled to tube I through the usual coupling condenser 1. Suitable plate voltages for the tubes may be supplied from a source designated 3+, and a- 11 Claims. (Cl. 250-36) audio transformer 8, and the audio oscillations generated by the circuit may be derived from the secondary of the said transformer across tersuitable grid biasing voltage for the tube V2 may be supplied from a source designated 0-. The output of the amplifier may be supplied. to an minals 9.

In accordance with thepresent invention, there is provided a novel feedback arrangement including a bridged T network designated generally reference character I0. This network comprises capacitance arms 01 and C2 bridged by a resistor R1, and a variable resistance R2 forming the upright of the T. One armof the network I 0 is connected to the anode or plate circuit of tube V2 by means of the conductor I I, so as to derive a portion of the audio output of the amplifier I. The other-arm of the network 10 is connected through a conductor I 2 to the-ungrounded end of a resistor R3 which is included in the cathode circuit of tube V1. The upright element R2 of the network I0 is connected through a conductor l3 to the ungrounded end of a variable resistor R4 which is included in the gridcircuit of the tube V1. The lattercircuit also includes a grid resistor l4.

As may be seen from the illustration, the bridged T network I0 and the associated resistors R3 and R4 provide degenerative and regenerative paths in the feedback arrangement. The degenerative or negative feedback path extends through the bridged arms of the network to the cathode resistor R3, while the regenerative or positive feedback path extends through the upright element R2 of the network to the variable grid resistor R4. The variable resistor R2 constitutes the tuning element by which the frequency of oscillation may be continuously varied overthe operating frequency range. If the elementsci, C2 and R2 and the associated positive feedback path were removed, negative feedback would take place through the resistors R1 and R3, and the magnitude of such feedback would be substantially independent of frequency. With the aforementioned frequency selective elements and the positive feedback path present, however, there is less negativefeedback at the frequency to'which the circuit is tuned. The variable resistor R4 serves to control the magnitude of the positive or regenerative feedback, and by proper adjustment of this resistor the circuit may be caused to oscillate at the frequency to which it is tuned. In accordance with the usual procedure for operating oscillators of this general type, the regeneration control elemaximum and minimum frequency values are related by ratios as high as ten-to-one or greater. The particular frequency band or range over which the circuit is adapted to operate in any instance will depend upon the values of theelemerits of the bridged T network and the feedback resistors R3 and R4. With a given set of values, the frequency may bevaried overlthe permissible range by varying'the resistor R2 irom its minimum value to its maximum value, or vice versa. If desired, provision may be made ior band switching by changing the valuesof the condensers C1 and C2. As will be apparentlto those skilled in the art this could be accomplished by substitution of various condensers-by m ans of w a suitable switching -a .-rrangement.

By employing i se es rsto a the re ue cy control ment-R h ire u c ren e ma b substantially increased, but at the sametimethe use of a linear resistor tends toycrowd-the frequencies at the end of the band where a. larg,e change in frequency-is effected-by a small clojange in resistance. In such case iit gnaybenecessary to employ logarithmic resistance variation to spread the frequencies. While a srnallerlinear resistance affords a lesser frequeney range, it gives a better Espre ad ot lfrequen oiesat the high frequency end of the band.

It is also characteristic ofthe circuit illustrated that the frequency control element Ratendsto affect the amplitude of 1the,generatedloscillations as the frequencylis varied. .QSiich yariationsi-n amplitude as may 'be thus caused, "may re ubstantially eliminated fiby employin .a suitable A. V. C. arrangement.

While, as previously statedftheparticular -form of the audio-amplifier 2| "i's ,not a"ffeatnre of the present invention tne two-stage a miner illustrated may comprise avwv inpu ii jbe', and. a 6K6 output tube, and the circuit elements .may have thefollowing values:

By w'ay'o'f example, the; elements d'f t i efiback circuits m'aybe asiolloyvs:

I Resistor R1 -'-ohms 240;'Q0 Resistor Rt"- nee-u zsgs'o'o ResistorRa -'do 680 Resistor R4 -do @100 Con'densers CrandCz;eachsmicrofarali 2002 tended to represent .a two-stage resistance coupied amplifier such asshowndn Fig-1. other eleinentsof- Fig. 2I correspond'ing to those of Figgl are designatedcorrespondingly. In ,this instance, however,,an audio signal to be ga-mplified is supplied:to?therampliiiergbywayoftthe -in-.

4 put terminal 16 and the coupling condenser H. In operation, the regeneration control element R4 is adjusted to a point just below that at which oscillation will take place. In this instance, the control element R2 serves, together with the associated elements, to controlthe frequency of maximum response. The feedback arrangement effects frequency-selective regenerative action which improves the response or gain at the resonant rr quency and also produces a sharp cut-oil above and'below the resonant frequency. By adjustment of the resistor R2, the'band resonant irequenoy maybevaried over a substantial range. In this embodiment of the invention, a suitable A. V.'.C.sy-stem may be employed to maintain the amplifier gain just below the point of oscillation.

.It is also possible by means of this circuit to change'the frequency range or band by changing thervalues of the elements in the feedback circuits.

Alth u c rtai emb dimsa 9 t inve tion have beenillustr'atedfor the purpose of disclosure, the invention is not l mited thereto, but is capable of other embodimentsand modificatiqns.

.1. In avacnum tube circuit, a vacuum tube mpl fie w os ute eme t ic mpr s ce trol grid and a cathode a ,ieedback connection between the amplifier output and said -lcathode, said connection including a resistor and apair of serially-connected capacitors i-n panallel-w-ith said resistor, and an auxiliary feedback connection between the junction of said capacitors and said grid, said auxiliary connection including a variable resistor.

2. In a yacuu n tube circuit, .a vacuum tube amplifier whose input elements include acontrol grid .and acathode, a, variable resistor connected between saidg-rid and ground, a second resistor connected between said cathodeand ground, -9, bridged T network including anaadjustable element, means connecting thearms of said T network between the output of said amplifier and the cathode end of said second :resistor, and means connecting :the upright of said ;T network to the grid end of said first resistor.

3. oscillator oircuit, comprising an amplifier 1 including at least two 1 vacuum tubes each having a control grid and a cathode, a variable esi t con ected betw en th rid of the :first tube and-ground a second resistor connected between the cathode of the first ,tubeand ground, a b i e T netw rki cludms adjustable-element, means a connecting the arms :of :said T :net- .workbetweeii the output of said amplifier :and the cathode end, of :said second resistor, and meansconnecting the tupright' of' said' T network to the grid 'endofsaid f rst resistor, g

A. An oscillator circuit qas'defined in claim 3, wherein said .net workdncludes capacitance arms bridged by a resistor, and the upright of the T is a yariable resistor. l e

5. In combina-tion, 5a vacuum tube amplifier having rid end-cathode inp1 it elements, and a feedbackcoupling network; connectedbetweemthe output and input of said amplifier, said network being er the bridgedT type; and being arran to provide negative and positive feedback paths, the bridged .arms of ;th e El metwork beingiine eluded in aieedback connection extending to said cathode -;element, and the' lupright of the netwerkbeingjncluded in an auxiliary feedback a hren eedins. i sa grid ment, none Of .th elements .ofrsaid network being adjustable to relatively vary the negative and positive feedback actions.

6. An oscillator circuit, comprising a vacuum tube amplifier having grid and cathode input elements, and feedback coupling means between the output and input of said amplifier, said coupling means including a bridged T network arranged to provide negative and positive feedback paths, said T network including capacitance arms bridged by a resistor, said arms being connected to the amplifier output and to said cathode element, respectively, and the upright of said T network being connected to said grid element, said upright including a variable resistor adapted to relatively vary the negative and positive feedback actions and to effect oscillation of the circuit at a desired frequency.

7. A frequency-selective audio amplifier circuit, comprising a vacuum tube amplifier having grid and cathode input elements, and feedback coupling means between the output and input of said amplifier, said coupling means including a bridged T network arranged to provide negative and positive feedback paths. said T network including capacitance arms bridged by a resistor, said arms being connected to the amplifier output and to said cathode element, respectively, and the upright of said T network being connected to said grid element, said upright including a variable resistor adapted to relatively vary the negative and positive feedback actions.

8. In combination, a, vacuum tube amplifier having at least an output electrode and a pair of input electrodes, a bridged T network, means including said network for coupling said output electrode and one of said input electrodes in degenerative phase relation, means including said network for coupling said output electrode and the other of said input electrodes in regenerative phase relation, and means for varying a parameter of said network.

9. In combination, a vacuum tube amplifier having at least grid and cathode input electrodes and an output anode, a bridged T network, means including said network for coupling said anode and said cathode in degenerative phase relation, means including said network for coupling said anode and said grid in regenerative phase relation, and means for adjusting a circuit element of said network.

10. An oscillator circuit, comprising a vacuum tube amplifier having at least an output electrode and a pair of input electrodes, a bridged T network, means including said network for coupling said output electrode and one of said input electrodes in degenerative phase relation, means including said network for coupling said output electrode and the other of said input electrodes in regenerative phase relation, and means for varying a parameter of said network.

11. An oscillator circuit, comprising a vacuum 1 tube amplifier having at least grid and cathode input electrodes and an output anode, a bridged T network, means including said network for coupling said anode and said cathode in degenerative phase relation, means including said network for coupling said anode and said grid in regenerative phase relation, and means for adjusting a circuit element of said network.

JAMES C. DUNN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,897,639 Kreer Feb. 14., 1933 2,173,427 Scott Sept. 19, 1939 2,186,571 Beale Jan. 9, 1940 2,268,872 Hewlett Jan. 6, 1942 2,298,177 Scott Oct. 6, 1942 2,319,965 Wise May 25, 1943 2,386,892 Hadfield Oct. 16, 1945 

